The effects of removing illumination gradients on the spatial distribution characteristics of visible fat within beef longissimus dorsi muscle were investigated. Gradients were assumed to be slowly varying and were removed by subtracting a smoothed version of the image from the original. Two types of filters were used to smooth the images: a linear lowpass filter and a greyscale morphological opening. The effects of the two filters on the binary images after thresholding using a Boolean random set model were investigated. Results showed that the choice of filter significantly influenced model parameter estimates. It was concluded that the morphological filter provided the most consistent means of eliminating adverse effects of illumination gradients. 1.